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1 First Name: Family Name: Student Number: Class/Tutorial: WOLLONGONG COLLEGE AUSTRALIA A College of the University of Wollongong Diploma in Information Technology Final Examination Spring Session 2008 WUCT121 Discrete Mathematics This exam represents 60% of the total subject marks Reading Time: 5 minutes Time allowed: 3 Hours DIRECTIONS TO CANDIDATES Refer to Page 2 ITC Education Ltd trading as Wollongong College Australia CRICOS 02723D ABN WCA-WUCT121-EXSF Page 1 of 9

2 DIRECTIONS TO CANDIDATES 1. Total number of questions: All questions are to be attempted. 3. All answers must be written in the Examination Answer Booklet supplied. 4. Questions are of equal value. 5. All working must be clearly shown. 6. Non-programmable calculators may be used. 7. No other examination aids are permitted. 8. Electronic dictionaries are not allowed. 9. Non-English speaking background students may use an approved English to Foreign language translation dictionary 10. The examination paper must not be removed from the examination room and must be submitted with the examination answer booklet. 11. During the 5 minutes reading time, you may make notes on the examination paper only. Answer Booklets must remain closed. WCA-WUCT121-EXSF Page 2 of 9

3 START OF EXAM Question 1 (a) Using the quick method, prove that the following statement is a tautology: (~ p ~ q) ~ ( p q) (b) Using full truth tables, determine whether the following statement is a tautology, a contradiction, or is contingent: ~ (( q ~ p) ( p ~ q)) ~ (~ p ~ q) [12 marks] (c) Write the following statement in predicate calculus notation using quantifiers and variables: Some students cannot correctly answer all questions in this exam. [8 marks] (d) Write in simple English without using quantifiers or variables: "x Œ +, $y Œ +, (y < x), where + is the set of positive Real numbers. (e) Write down, then simplify, the negations of each of the statements in parts (c) and (d) using; (i) Predicate calculus notation using quantifiers and variables. (ii) Simple English without using quantifiers and variables. (f) For the following sentence: The statement that all statements are sentences but not all sentences are statements is false. (i) Determine if it is a statement; and explain why or why not. (ii) If it is a statement, determine whether it is true or false, giving reasons for your answer. WCA-WUCT121-EXSF Page 3 of 9

4 Question 2 (a) Using a direct proof, prove the following statement: 2 + For any real number x, 20( x 1) 4 x 5 (b) Briefly explain how the following tautologies may be used in the method of proof by contradiction: (i) (~ p ( q ~ q)) p (ii) ( p q) (~ q ~ p) (c) Using proof by contradiction or otherwise, prove the following statements: (i) For all integers n, if n 3 is even then n is even. (ii) There is no largest even integer. [8 marks] [8 marks] (d) Consider the following statements: P : Q : R : 2x + 1 = 3 2x = 2 x = 1 If we assume that P is a true statement, explain how these statements, together with Modus Ponens and the Law of Syllogism, can be used to prove that R is also a true statement. [10 marks] (e) Complete the following tautology, which is used in the method of proof by cases: K KKKKKKKK (( p r) KK( q r)) (f) Using proof by cases or otherwise, prove the following statement: For all integers n, 2 n 2 1 is odd. [10 marks] WCA-WUCT121-EXSF Page 4 of 9

5 Question 3 (a) Use Mathematical Induction to prove that 5 n 1 is divisible by 4 for all n Œ Õ. (b) (i) Use the Euclidean Algorithm to find gcd(126, 39). (ii) Find m, n Œ Ÿ such that 126m 39n = gcd(126, 39). (c) (i) Showing all steps clearly, use the Sieve of Eratosthenes to find all the prime numbers between 2 and 52. [8 marks] (ii) Write down all the twin primes between 2 and 52. (d) Use the Generalized Pigeonhole Principle to justify your answer to the following question: In a group of 2000 students, must at least five have the same day of the year as their birthday? (e) Find a value of x such that x = 5 7 (mod) 17 and 0 x < 17. (f) Write down the complete set of residue classes modulo 11, Ÿ 11. (g) Express each of the elements [ 3], [7], [44] Œ Ÿ 11 as [x] Œ Ÿ 11, 0 x < 11. (h) Let S Ÿ 11 be given by S = {[1], [4], [7]}. (i) Construct the multiplication table for S. [6marks] (ii) Using this table, or otherwise, determine the identity for multiplication and the multiplicative inverses of all the elements in S which have them. WCA-WUCT121-EXSF Page 5 of 9

9 (e) For the graph G given in question (d) above, either write down a path with the specified properties, or explain why no such path exists. (i) A simple path of length 3 from v 1 to v 2. (ii) A path of length 4 from v 1 to v 3. (f) Is the graph G given in question (d) above connected? If so, what is the least number of edges you must remove to make the graph disconnected? Give an example. (g) If the edges in graph G have the following weightings, use Kruskal s Algorithm to find a minimum spanning tree for G: e =, e = 7, e = 9, e = 2, e = 6, e = 3, e = (h) Label the vertices and edges of the following graphs in any convenient way and show whether or not they are isomorphic. END OF EXAM WCA-WUCT121-EXSF Page 9 of 9

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